Flat plate solar collectors have been generally known for many years. Such flat plate solar collectors typically consist of a solar radiation absorbing member heated by solar radiation impressed thereon, a heat transfer fluid which is passed into contact with the solar radiation absorbing member to transfer the heat from the solar radiation absorbing member into the heat transfer fluid, and one or more generally transparent covers over the solar radiation absorbing member which admits incoming solar radiation while retarding thermal heat loss from the solar radiation absorbing member. Also, typically, an insulated enclosure is provided around the bottom and sides of the collector to minimize thermal heat loss. The solar radiation absorbing member is typically a metal plate painted or plated black such that about 90% of the solar radiation incident upon it will be absorbed and converted to heat therein.
One type of these prior art solar collectors employs the technique of trickling water over the solar radiation absorbing member which is tilted at an angle and generally faces the sun. This type collector was proposed in U.S. Pat. No. 1,101,001 and is commonly known as the trickle-type collector. This type collector has two serious drawbacks. The first drawback is that, when water is trickled over the solar radiation absorbing member, it may not wet the entire area of the member surface but instead runs down in streams or channels to seriously reduce the heat collection efficiency of the collector. The other problem with this type collector is that temperatures are readily reached in the solar radiation absorbing member to cause the water flowing along the member to be partially evaporated and then condensed on the cooler cover to thereby greatly increase the thermal heat loss due to the heat pipe effect and also to limit the amount of solar radiation that can reach the solar radiation absorbing member to be transferred to the water flowing over the solar radiation absorbing member. Attempts to solve some of the problems with the trickle-type collectors are shown in U.S. Pat. Nos. 3,145,707 and 3,215,134 which add an additional transparent inner flexible sheet between the collector cover and the solar radiation absorbing member and the inner sheet to prevent condensation on the inside of the collector cover. This type collector has become known as the Thomason collector. The Thomason collector, however, still suffers from the disadvantage that the water usually will not wet the entire area of the solar radiation absorbing member and continues to run down the solar radiation absorbing member in streams or channels to produce a continued reduction in collector efficiency. The use of the inner flexible sheet has also increased the original manufacturing cost of the collector.
Because of the inherent problems of the trickle-type and Thomason collectors, the closed channel type of solar collector has received more attention and consists of a metal coil, usually copper, which is fastened or soldered to the solar radiation absorbing member to serve as a passageway through which water or some other heat transfer fluid may be circulated to transfer the heat from the solar radiation absorbing member to a storage tank or some other heat operated device. Alternatively, the solar radiation absorbing member has been made of two channelled metal plates bonded together so as to provide a circuitous passageway between the plates through which the heat transfer fluid can flow. While these channel type collectors have proved to be quite successful in operation, they are relatively costly to manufacture thereby seriously limiting their commercial use. Additionally, these collectors also require considerable amount of supplemental power to provide the relatively high pumping pressure required to circulate the heat transfer fluid through the enclosed passageways.
Anothr factor which is seriously limiting the use of these prior art solar collectors is that each collector is usually made in a fixed size or must be custom made to any particular size thereby maintaining a relatively high cost of installation.